News and Perspectives from Pacific Gas and Electric Company

PG&E customers can drive electric vehicles (EVs) with the greenest of consciences, knowing that the power they draw comes from some of the cleanest sources in the nation. But residents of much of the rest of the country face a much more ambiguous choice when it comes to selecting a clean vehicle.

Some mistakenly believe that charging an electric vehicle at night is always preferred. Some researchers now say that "when” you charge is as important as “where.” (Photos by Matt Nauman.)

Experts have long pointed out that where you charge your EV has a big impact on its environmental footprint. Thus electric vehicles are no panacea In China, which burns vast amounts of dirty coal to make its power grid hum. The same goes for parts of the United States dominated by coal-fired power plants.

Now, compounding the problem, a new working paper by three economists points out that when you charge matters a lot for the environment, too.

At first glance, that hardly seems like news. PG&E has long structured its special EV charging rates to incent customers to plug in at night, when other demands on the power grid are low. Charging after dark avoids overstressing the grid during afternoon peaks, and it reduces the need to call on costly and polluting fossil-fueled “peaker plants” to meet the needs of these vehicles.

Grid geography matters

Many authorities, like the Environmental Law and Policy Center in the Midwest, mistakenly believe that charging at night is always “better for the environment.”

But, hard as it may be to believe, what makes sense in California and other Western states doesn’t hold true everywhere, according to economists Joshua Zivin (UC San Diego), Matthew Kotchen (Yale), and Erin Mansur (Dartmouth).

California’s clean grid makes night charging of EVs best for car owners and the environment. In the Midwest, though, night charging might mean drawing power from coal plants.

In the upper Midwest, for example, cars charging in the wee small hours of the morning will draw their extra power from coal-fired plants that run 24/7. Every kilowatt-hour of energy they suck from the grid will result in about 2.8 pounds of CO2 emissions, according to the new analysis.

That’s 3.5 times the level of emissions from the same car charging in the Western United States, where generation is much cleaner, thanks in part to plentiful hydropower.

Now move the charging time to 7 p.m. and matters change markedly in the Midwest, where utilities call on previously idle natural gas-fired plants to help meet the afternoon peak. Because gas is cleaner than coal, CO2 emissions fall as low as 1.7 pounds per kilowatt-hour, still a bit more than twice as much as in the West but far better than late at night.

In fact, averaged across the United States, peak afternoon hours tend to be cleaner than night-time for charging. Unfortunately, peak hours also tend to be the most expensive in all parts of the country, because utilities have to call on higher-cost generators to meet the extra demand. So we in California are fortunate to align low costs and low emissions at the same times.

Move to natural gas changing the equation

As utilities in the Midwest and other regions retire older coal plants in favor of less-polluting natural gas-fired plants, EVs will become a better choice for many environmentally conscious customers there. In the meantime, car buyers in such regions should consider one of the many high-mileage hybrid vehicles, which emit less CO2 than EVs powered by coal.